1. Field of the Invention
This invention relates to pressurized water reactors and, more particularly, to a top end support for the water displacement rod guides positioned within the inner barrel assembly of a pressurized water reactor.
2. State of the Prior Art
Certain advanced designs of nuclear reactors incorporate at successively higher, axially aligned elevations within the reactor vessel, a lower barrel assembly, an inner barrel assembly, and a calandria, each of generally cylindrical configuration, and an upper closure dome. The lower barrel assembly may be conventional, having mounted therein, in parallel axial relationship, a plurality of fuel rod assemblies which are supported at the lower and upper ends thereof, respectively, by corresponding lower and upper core plates. Within the inner barrel assembly there is provided a large number of rod guides disposed in closely spaced relationship, in an array extending substantially throughout the cross-sectional area of the inner barrel assembly. The rod guides are of first and second types, respectively housing therewithin reactor control rod clusters (RCC) and water displacer rodlet clusters (WDRC); these clusters, as received within their respectively associated guides, generally are aligned with the fuel rod assemblies.
The calandria includes a lower calandria plate and an upper calandria plate. The rod guides are secured in position at the lower and upper ends thereof, respectively, to the upper core plate and the lower calandria plate. The inner barrel assembly thus is defined in axial height between a first plate of a lower elevation comprising the upper core plate and a second plate of a higher elevation comprising the lower calandria plate. Within the calandria and extending between the lower and upper plates thereof is mounted a plurality of calandria tubes in parallel axial relationship and respectively aligned with the rod guides. A number of flow holes are provided in remaining portions of the calandria plates, intermediate the calandria tubes, through which passes the the reactor core outlet flow as it exits from its passage through the inner barrel assembly.
In similar parallel axial and aligned relationship, the calandria tubes are joined to corresponding flow shrouds which extend to a predetermined elevation within the dome, and which in turn are connected to corresponding head extensions which pass through the structural wall of the dome and carry, on their free ends at the exterior of and vertically above the dome, corresponding adjustment mechanisms. The adjustment mechanisms have corresponding control lines which extend through the respective head extensions, flow shrouds, and calandria tubes and are connected to the respectively associated clusters of RCC rods and WDRC rods, and serve to adjist their elevational positions within the inner barrel assembly and, particularly, the level to which same are lowered into the lower barrel assembly and thus into association with the fuel rod assemblies therein, thereby to control the activity within the core.
A critical design criterion of such reactors is to minimize wear of the rodlets in the rod guides, and thus to reduce or eliminate the factors which produce wear, such as flow induced vibration. Because of the relatively dense packing of the rod guides within the inner barrel assembly, it is critical to maintain substantially uniform distribution of the outlet flow from the reactor core, and an axial direction of that flow through the upper barrel assembly. Even if a uniform, axial flow of the core outlet is achieved, the effects of differential pressure and temperature across the array of rod guides or an individual rod guide can produce significant reaction loads at the support points, or support connections, for the rod guides. These reaction loads, coupled with the flow induced vibration, create a high potential for wear at the support points for the rod guides as well as the rodlets. Additionally, the provision of the calandria, and particularly the lower plate thereof, presents an interface with the top end of the rod guides which does not exist in conventional pressurized water reactors. That interface must be capable of accommodating differential thermal expansions between the lower calandria plate and the inner barrel in order to prevent large thermal stresses from developing. Furthermore, the lower calandria plate and the upper core plate are essentially structurally independent; therefore, vibration of the internals can result in significant relative movement between the supporting connections of the rod guides at their lower and upper ends to the respective upper core plate and the bottom calandria plate. The wear potential under these circumstances is great.
Thus, split, or two-pin connections of conventional types are inappropriate for use as the supporting connections for the top ends of the rod guides since they have too much torsional stiffness. If used, they would wear rapidly, with the result that the top ends of the rod guides would become loose. Rod guides having such loose top end connections are unacceptable because of the rapid rate of wear of the rodlets which would result. Other known mounting devices as well are inappropriate. For example, leaf springs cannot be used to support all of the rod guides because sufficient space is not available within the inner barrel assembly to provide leaf springs of the proper size for the large number of rod guides which are present, even if high strength material is used for the leaf springs.
Beyond the unsuitability of existing, known structural support arrangements, further factors must be taken into account in the consideration of possible designs for the support of the top end of the rod guides within the inner barrel assembly. For example, both the RCC and the WDRC rod clusters should be removable without requiring that the guides be disassembled. This requirement imposes a severe space limitation in view of the dense packing of the guides and their associated rod clusters within the inner barrel assembly. For example, in one such reactor design, over 2,800 rods are mounted in 185 clusters, the latter being received within a corresponding 185 guides. The space limitation is further compounded by the requirement that unimpeded flow holes must be provided in the calandria plates for the core outlet flow. While these foregoing factors severly restrict the available space envelope in the horizontal cross-sectional dimension of the inner barrel assembly, axial or vertical limitations on this space envelope must also be considered. For example, the presence of the support members should not require any increase in the height of the vessel. From a maintenance standpoint, the support members should be replaceable without undue effort. Additionally, the assembly of the calandria must be accomplished without access to the support region.
While the supports for the rod guides must therefore satisfy a wide range of structural and functional requirements relating to, or imposed by, the inner barrel assembly itself, a further critical requirement is that the wear potential of the support structure itself must be minimized. This is a critical requirement in view of the potential for intense vibration arising out of the core outlet flow and the development of contact forces due to differential pressure and steady state and transient temperature conditions across the array of rod guides and the individual rod guides.
Conventional reactor designs do not present the support problems attendant the dense packing of rod guides and associated rod clusters in advanced reactor designs of the type herein contemplated. Thus, there is no known solution to the problems of adequately supporting the rod guides, consistent with the requirements and taking into account the environmental factors which exist in operation of such reactors as hereinabove set forth.